The correct answer is A) dip the blade of your oar into the water vertically and hold it perpendicular to the keel line. When the coxswain gives the command "Hold water," the expected response is to dip the oar vertically into the water and hold it perpendicular to the keel line. This action serves to stop the forward movement of the lifeboat and maintain its position in the water. The other options, such as completing the stroke and raising the oar out of the water, would not effectively hold the boat's position as required by the coxswain's command.